1. Field of the Invention
The present invention is directed to a control system for a sodium-sulfur battery, which is capable of properly controlling the charge and discharge of a sodium-sulfur battery, adjusting the temperature thereof and the like, and a sodium-sulfur battery being provided with such a control system.
2. Description of the Related Art
A sodium-sulfur battery (hereinafter, sometimes referred to as a NaS battery) comprises a plurality of sodium-sulfur cells connected with each other, and being used for various practical applications. Such applications include, for example, being used as a power storage system for leveling electric power demand so as to cope with a large difference in demands during daytime and nighttime, particularly, or being used as a peak-cut power supply system for supplying electric power for time periods during which power demand sharply increases in the summer season, or being used as an emergency power supply system in natural disaster situations.
The NaS battery is used, for example, as a NaS battery power storage system in which a circuit is formed between a NaS battery, an AC/DC converter, and other components; said NaS battery being constituted from the components produced in the manner discussed below. Firstly, a NaS battery string (a group of cells) is formed by connecting a plurality of cells in series, then a NaS battery block is formed by connecting a plurality of thus formed NaS battery strings in parallel to each other. Thereafter, a plurality of a plurality of thus formed NaS battery blocks are connected in series to form a NaS battery module (hereinbelow, sometimes simply referred to as a battery module), then a NaS battery is formed by connecting a plurality of thus formed NaS battery modules in series.
The NaS battery is a secondary battery in which molten metal sodium, as a cathode active material, and molten sulfur, as an anode active material, are arranged separately from each other, using a β-alumina solid electrolyte having a selective permeability toward sodium ions. The discharge of the NaS battery is done by the following manner. Molten sodium liberates an electron, and becomes a sodium ion. Thus formed sodium ion moves toward the positive electrode by passing through said solid electrolyte and then reacts with sulfur and electrons supplied from an external circuit to produce sodium polysulfide. On the other hand, the charge is done as a reverse process of the discharge; that is, a sodium and sulfur is formed as a result of reaction of sodium polysulfide with emission of an electron. In the viewpoint of charge/discharge efficiencies, preferably, the NaS battery is operated at a high temperature of 280° C. or more in consideration of the temperature characteristics of sodium ion conductivity with respect to β-alumina. However, the operating temperature of the NaS battery is limited due to the heat resistances of various components constituting the battery, and the like. Therefore, it is important to make the NaS battery charge or discharge while keeping an operating temperature within a predetermined range of from 280 to 360° C., for example.
Further, it is also important to properly control the charging and discharging operations of the NaS battery. For example, in each cell, an open circuit voltage at the depth of full-charge is kept constant at 2.075 V. As the cell is being discharged, the electromotive voltage gradually decreases. When the discharge is completed (the end of discharge), the open circuit voltage is set to substantially 1.82 V. However, for example, a voltage measured during the discharge is lower than the open circuit voltage by the product (voltage drop) of internal resistance and a discharge current. Therefore, during the operation of the NaS battery, it is necessary to compensate for the voltage drop by adding a voltage equal to the voltage drop to the measured voltage to obtain an open circuit voltage during the discharge and then, detect the end of discharge.
A conventional control system, which has been used to implement the above proper operation of the NaS battery, comprises individual module control devices (hereinbelow, sometimes referred to as a module controller), provided for each of the battery modules laid in a frame for NaS battery, and a general-purpose control device, such as a sequencer, installed on a control panel provided independent of the frame for NaS battery. Each module controller measures a voltage and a temperature of each battery module to monitor the operating state thereof and also turns on or off a heater provided with each battery module to adjust the operating temperature of the NaS battery. For example, the NaS battery discharge current is measured using a current measuring function of the general-purpose control device, or sequencer. In the sequencer, a voltage drop is calculated to determine a discharge cutoff voltage. Thus, the end of discharge in the NaS battery can be detected. The cutoff voltage means a reference voltage is used to determine the end of charge or discharge of the NaS battery.
An illustrative showing of a conventional control system for a NaS battery is given in FIG. 2 in which five battery modules are connected in series. The control system comprises module controllers 26, one of which is provided for each NaS battery module 24 in a battery frame 21 of a NaS battery 22, and a sequencer 23 is provided on a control panel 31 that is independent of the battery frame 21 of the NaS battery 22. Each module controller 26 has a temperature measuring element, a voltage measuring element, and a heater on or off control, to control the operating temperature of the NaS battery 22 and to which heater power 27 is supplied through a heater power supply line 127. Each module controller 26 also has transmitting/receiving element, in accordance with RS-422 standard or the like, to transmit measurement data and a signal indicative of the operating state of the corresponding heater 25 to a control unit 28 of the sequencer 23. The sequencer 23 receives temperature and voltage information indicating the state of each battery module from each module controller 26 through the transmitting/receiving element, in accordance with RS-422 standard or the like. The sequencer 23 includes a measurement unit 29 for measuring an output current (discharge current) of the NaS battery 22 comprising the battery modules 24 connected in series. These measurement values and information are displayed on a display (not shown) provided on the control panel 31 and are also transmitted as external signals 20. The external signals 20 can be confirmed by remote monitoring hardware through, for example, a data link.
The above conventional control system has sufficiently served for the purpose of controlling the NaS battery in the development stage for practical application. However, with the current demand for the NaS battery as, for example, a power storage system being widespread, an improvement in long-term reliability, a reduction in the trial operating costs of equipment and a reduction in the time required for design and manufacture are further desired in the market. Then, the following problems come to exist.
(a) Misdetection at the End of Charge/Discharge
Hitherto, for example, when the end of discharge is detected, a discharge cutoff voltage VL is obtained by the following expression (1) using a discharge current Id measured by a current measurement unit of a sequencer, internal resistance R of each battery module, and a temperature coefficient Kt, which fluctuates with the operating temperature T:VL=VO×n−Id×R×Kt   (1). The discharge cutoff voltage VL is compared to the actual operating voltage V of each battery module, which is being measured by each module controller and then transmitted to the sequencer. When the following expression (2) is satisfied, it was judged to be the end of discharge:VL>V   (2). In this instance, reference symbol VO denotes an open circuit voltage of each cell just before sodium at the negative electrode becomes short, and this open circuit voltage VO is usually about 1.82 V. Reference symbol n denotes the number of cells included in each battery module. In other words, the discharge cutoff voltage VL indicates an operating voltage at the theoretical end of discharge of the NaS battery. However, since the operating voltage V data is transmitted from the module controller to the sequencer and the comparison and the end of discharge determination are then performed, a transmission delay occurs. Accordingly, the operating voltage V is not accurately synchronized with the discharge voltage Id due to the time delay between calculating the discharge cutoff voltage VL, from the discharge current Id, and comparing VL to the module operating voltage V. Thus, some time lag always exists in the determination of the end of discharge. Consequently, a percentage of the charged power cannot be effectively used.
When the end of charge is detected, a charge cutoff voltage VH is calculated by the following expression (3) using a charge current Ic measured by the current measurement unit of the sequencer and the internal resistance R of each battery module:VH=(VI+α)×n−Ic×R   (3). The charge cutoff voltage VH is compared to the actual operating voltage V of each battery module, the voltage V being measured by each module controller and being then transmitted to the sequencer. When the following expression (4) is satisfied, it was judged to be the end of charge:VH<V   (4). In this instance, reference symbol VI denotes an open circuit voltage of each cell at the end of charge, and this open circuit voltage VI is generally 2.075 V. Reference symbol n denotes the number of cells included in each battery block. Reference symbol a denotes polarization resistance generated at the end of charge. This polarization resistance is generally 0.05 to 0.15 V. In other words, the charge cutoff voltage VH indicates a voltage obtained by adding the polarization voltage to the open circuit voltage at the theoretical end of charge of the NaS battery. However, due to a transmission time delay, as in the case of discharging, the operating voltage V is not accurately synchronized with the charge current Ic. Thus, there is observed a delay between calculating the charge cutoff voltage VH from the charge current Ic and comparing VH to the module operating voltage V. Thus, some time lag exists in the determination of the end of charge. Accordingly, charging is not sufficient and the full capacity of the battery cannot be effectively used.(b) Fluctuation in Power Consumption of Heaters
A three-phase, three-wire AC power supply is used as a power supply for heaters. The heaters, provided for each of the respective battery modules, are connected to each other in balance so that each heater serves as a rated line load between two lines. For example, 10 kW is applied as a load between R-phase and S-phase, 9 kW is applied as a load between S-phase and T-phase, and 10 kW is applied as a load between T-phase and R-phase. Inherently, the heaters serve as temperature raising elements for setting the operating temperature of the NaS battery in a proper range. The heaters are individually turned on or off in accordance with the state of the NaS battery. Each conventional module controller turns each heater on or off independent of the other module controllers. Accordingly, all of the heaters are simultaneously turned on or off at a certain probability. Since a fluctuation in the power consumption of the heaters is very large, each transformer and circuit breaker needs a capacity corresponding to those of the heaters. Further, the voltage fluctuates due to changes in the heater loads.